Currently under investigation is the interaction of bovine brain phospholipid exchange protein with liposomes (single bilayer vesicles) prepared from a variety of natural and synthetic lipids. The interaction is monitored by the ability of the protein to catalyze the transfer of phosphatidylinositol or phosphatidylcholine from rat liver microsomes to these liposomes. In addition, the kinetic parameters (Km and Vmax) for phospholipid transfers are determined. The results thus far obtained clearly indicate that the physical and chemical character of the liposome surface is critical to protein-membrane interaction. The presence of negatively-charged lipids in the liposome has no effect on phospholipid transfers, the one exception being phosphatidylinositol which is the preferred substrate for this exchange protein and for which the protein has the highest binding affinity. Positively-charged lipids tend to decrease protein-membrane interactions, while certain zwitterionic phospholipids significantly enhance such interactions. Liposomal cholesterol has no effect on this microsome-liposome transfer system. The study will continue with an examination of liposomal phospholipid fatty acid composition, with particular emphasis on thermotropic phase transitions and membrane microviscosity.